S.-K. Choi et al., Belle Collaboration

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1 S.-K. Choi et al., Belle Collaboration
Observation of a Narrow Charmoniumlike State in Exclusive B± → K± π+ π− J/ψ Decays S.-K. Choi et al., Belle Collaboration Phys. Rev. Lett. 91, (2003) Contents 1. Introduction 2. KEKB accelerator 3. Belle detector 4. Analysis 5. Result 6. Summary Shibata lab. 12B16340 Yuuki Fujii

2 1. Introduction The purpose of Belle experiment is to observe CP violation using the difference between B meson decay and anti-B meson decay. Another purpose is to perform hadron spectroscopy. Belle collaboration produced many charged and neutral B mesons. B mesons have many decay modes. This paper reports B± → K± π+ π− J/ψ decays. J/ψ π+ B+ π- K+ e- e+ J/ψ π+ B- π- K-

3 2. KEKB accelerator KEKB accelerator is the asymmetric e+e− collider at KEK in Tsukuba. e− energy : 8 GeV e+ energy : 3.5 GeV center-of-mass energy : = 10.6 GeV KEKB ring Belle e- e+ ⇒ LINAC 𝑠 1km KEKB collider produces a lot of B meson and anti-B meson pairs. Therefore, this collider is called B Factory. KEKB collider achieved the luminosity of 2.11 × cm −2 s −1 in June 2009.

4 3. Belle detector Belle detector is composed as follows from inside to outside: Silicon Vertex Detector (SVD) Central Drift Chamber (CDC) Aerogel Cherenkov Counters (ACC) Time-of-flight counters (TOF) Electromagnetic calorimeter (ECL) KL meson and muon detector (KLM) 7 m 7.7 m Nucl. Instrum. Methods A 479, 117 (2002) These detectors determine momentum, energy and trajectory of particles. ⇒

5 4. Analysis Belle collaboration determined invariant mass of π+ π− J/ψ in B± → K± + π+ π− J/ψ decay process. The invariant mass is calculated from energy and momentum of decay particles: 𝑀 = 𝑖 𝐸 𝑖 2 − 𝑖 𝑝 𝑖 2 J/ψ meson decays to e+e- or μ+μ-. (J/ψ : charmonium state. It is bound state of charm quark and anti-charm quark. ) J/ψ 𝑐 𝑐 Invariant mass of lepton pair was evaluated first. Lepton pair was identified as J/ψ by 𝑀 ℓ + ℓ − 𝑀 ℓ + ℓ − Invariant mass was evaluated next. 𝑀 π + π − ℓ + ℓ − 𝑀 ℓ + ℓ − : invariant mass of J/ψ (ℓ+ℓ- : e+e- or μ+μ-) 𝑀 π + π − ℓ + ℓ − : invariant mass of π + π − J/ψ The mass difference is plotted in next page. 𝑀 π + π − ℓ + ℓ − − 𝑀 ℓ + ℓ −

6 Monte Carlo simulation
5. Result B± → K± + π+ π- J/ψ There are two peaks at GeV and GeV in real data. Monte Carlo simulation is smooth except for the GeV peak. 0.589 GeV peak corresponds to ψ′ mass 　　　　　 (ψ′ : an excited state of J/ψ) M π + π − ℓ + ℓ − − M ℓ + ℓ − GeV M π + π − ℓ + ℓ − − M ℓ + ℓ − GeV experiment data Monte Carlo simulation 0.775 GeV peak is small, but this signal has statistical significance of 10.3σ. The signal at GeV corresponds to 𝑀 = 𝑀 π + π − ℓ + ℓ − − 𝑀 ℓ + ℓ − + 𝑀 J/ψ = 3872 MeV. ( 𝑀 J/ψ : PDG value of J/ψ mass) Belle collaboration named this state as X(3872).

7 Belle collaboration first thought X(3872) was S=1, L=2, J=2 charmonium state ( ), ( 3D2 state ).
Mass value of this state has been theoretically calculated. c c 𝑀 𝐷 2 = 3810 MeV decay width of this state has been theoretically calculated. ( 　: photon , 　　: one of the charmonium states ) 3 𝐷 2 →γ χ 𝑐1 γ χ 𝑐1 Γ ( 𝐷 2 →γ χ 𝑐1 ) > 5 × Γ ( 𝐷 2 → π + π − J/ψ) Experimental values are not consistent with the calculated values. 𝑀 X(3872) = 3872 MeV Γ (X(3872)→γ χ 𝑐1 ) 0.89 × Γ (X(3872)→ π + π − J/ψ) < X(3872) is not a normal charmonium state.

8 Another interpretation
X(3872) X(3872) mass value is close to the ( ± 1.0 MeV) X(3872) is suggestive of a loosely bound multiquark “molecular state”, as proposed by a theory (M. Bander, G.L. Shaw, P. Thomas et al.) 𝑀 𝐷 𝑀 𝐷 ∗0 𝐷 0 𝐷 ∗0 c u c u 𝐷 0 𝐷 ∗0 u u c c K+ J/ψ B+ W+ s c c X(3872) b c u u π+ c d X(3872) u u d π− u u B+ → K+ + X(3872) X(3872) → π+ + π- + J/ψ X(3872) is not fully understood yet. Further study is needed.

9 6. Summary The purpose of Belle experiment is to observe CP violation using the difference between B meson decay and anti-B meson decay. Another purpose is to perform hadron spectroscopy. KEKB accelerator is the e+e- collider. Belle detector determines momentum, energy and trajectory. Belle collaboration observed a strong signal for a state. It decays to π+ π- J/ψ. M = 3872 MeV Belle collaboration named this state as X(3872). X(3872) mass value is close to the X(3872) is suggestive of a loosely bound multiquark “molecular state”, as proposed by a theory. 𝑠 = 10.6 GeV 𝑀 𝐷 𝑀 𝐷 ∗0 𝐷 0 𝐷 ∗0

10 補足

11 CP violation in B mesons
B中間子でのCP対称性の破れは、寿命の違いという形で現れ る。そのため、B中間子と反B中間子の崩壊までの時間を正確に 測定する必要があり、その測定のためにBelle実験は計画され た。 Belle実験は、2001年にB中間子における「CP対称性の破れ」が 存在することを発見した。

12 Spectroscopy スペクトロスコピーとは、特に励起状態の崩壊生成物を判 定する実験を意味する。それにより励起状態の性質や構成 要素の間の相互作用について知ることができる。 系が空間的に小さいほど系の励起エネルギーが大きく、励 起状態を生成するためには高エネルギーの粒子が必要と なってくる。

13 KEKB accelerator B+ + B- e+ + e- → γ* → Υ(4S) →
エネルギーが非対称・・・e- – 8 GeV , e+ – 3.5 GeV ⇒　生成物がe+の方向に運動量を持つ ⇒　相対論的効果により生成物の寿命が延びる 𝑠 = 𝐸 𝑒 − + 𝐸 𝑒 − 𝑝 𝑒 − + 𝑝 𝑒 = 𝐸 𝑒 − 𝐸 𝑒 − 𝐸 𝑒 + + 𝐸 𝑒 + 2 − 𝑝 𝑒 − −2 𝑝 𝑒 − 𝑝 𝑒 + − 𝑝 𝑒 ≃ 2 𝐸 𝑒 − 𝐸 𝑒 𝑝 𝑒 − 𝑝 𝑒 ≃ 4 𝐸 𝑒 − 𝐸 𝑒 + ∴ 𝑠 ≃ 4×8×3.5 ≃ 10.6 GeV B中間子生成までの過程 主な崩壊モード B+ + B- e+ + e- → γ* → Υ(4S) → u b b u B 0 + B 0 b b 質量： ± GeV d b b d

14 Luminosity of KEKB accelerator
1nb = 10 −33 cm 2

15 Belle detector Belle検出器は、B中間子崩壊に おけるCP対称性の破れを観測 するために設計された。
エネルギーが非対称なため、 衝突点が測定器の中心から少し ずれている。 ・荷電粒子の通過した位置を測定： Silicon Vertex Detector, Central Drift Chamber ・粒子のエネルギー測定：Electromagnetic calorimeter • 荷電粒子の識別： Central Drift Chamber, Aerogel Cherenkov Counters, KL meson and muon detector • 荷電粒子の飛行時間の測定： Time-of-flight counters

16 charmonium S: スピン (中間子なのでS=0 or 1)
L: 軌道角運動量 (L=0,1,2,…はそれぞれL=S,P,D,…で表される) J: 全角運動量 (|L-S| < J < L+S に従う) 2S+1LJのように表記される場合もある。 例えばJ/ψは 3S1という状態であり、量子数はそれぞれS=1, L=0, J=1である。 S=0のチャーモニウムの状態はηcと呼ばれるが、e+e-衝突で直接 作られることはない。

17 X(3872) Quantity ψ' X(3872) Signal events 489 ± 23 35.7 ± 6.8 M peak
± 0.2 MeV ± 0.6 MeV σ 3.3 ± 0.2 MeV 2.5 ± 0.5 MeV 𝑀 X(3872) = 𝑀 X measured − 𝑀 ψ′ measured + 𝑀 ψ′ PDG = ± 0.6(stat) ± 0.5(syst) MeV 𝑀 ψ′ PDG − 𝑀 J/ψ PDG = ± 0.13 MeV 𝑀 J/ψ PDG = ± MeV

18 Meson 粒子記号 構成クォーク 不変質量[MeV] B+ 5,279.15±0.31 K+ 493.677±0.016 π+
± J/ψ 3, ±0.011 D0 1, ± 0.17 D*0 2, ± 0.19 u b u s u d c c c u c u